Normally, focus knobs are designed as coaxially arranged rotating elements which are mounted aside the microscope, whereby the larger focus knob actuates the coarse focus and the smaller focus knob actuates the fine focus.
State of the art focus drives often have the disadvantage that conditionally to too small circumferential surfaces in the area of the coarse focus drive no safe gripping of the coarse focus drive is possible. Additionally, too extending fine focus drives interfere with the fingers while actuating the coarse focus drive. A too conical form of both rotating elements leads to an axial slipping of the actuating fingers. Also a too strong surface profile affects the tactile sensitiveness of the fingers tips disadvantageously in the long term, and the adhesive friction between the user's hand and the rotating element is unnecessarily reduced, which reduces the adjusting accuracy and additionally brings about the danger that the user unintentionally actuates the coarse focus while actuating the fine focus. The reason is that both rotating elements are arranged in a way to each other that the smaller focus drive penetrates the larger coarse focus drive as a cylinder and thus the abutting face of the coarse focus drive can be touched while the fine focus is being actuated, and thus the coarse focus drive is actuated unintentionally.
This occurs especially to microscopes with a non-mechanical transmission (in the case of a mechanical transmission the coarse focus moves so clumsily that this danger hardly exists). It happens more to those microscopes which have a motor control for the coarse and fine focus knobs. Concerning these microscopes, the focus drives move almost friction-free, which enables unintentional actuation, and which may lead to the damage to or even the demolition of the sample and/or of the optical system in the worst case. Concerning state of the art focus drives, this effect of the unintentional actuation of the coarse focus drive is also increased by that the rotating element for the fine focus drive by a haptic way indicates insufficiently how close already the fingers of the user are to the abutting face of the coarse focus drive.